1. Field of the Invention
The present invention relates to an electric instantaneous boiler which is used in the water heating operation for a shower or the like.
2. Description of the Prior Art
Conventionally, this type of electric instantaneous boiler, constructed as illustrated in FIG. 7, is disclosed in, for example, U.S. Pat. No. 4,358,665. Namely, valve 1 is opened and the pressure switch 2 is actuated by the water flow to turn the sheath heater 3 on. The water goes from the valve 1 to the lower portion of the tank 5 through the water pipe 4. The water goes to the upper portion of the tank 5 while being heated by the sheath heater 3 and flows into the inlet opening 6a of the hot water pipe 6 provided at the upper portion.
However, as seen in FIG. 9, the valve 1 is fully closed after a flow of 3 liters per minute at an input water temperature of 25.degree. C., and an output hot water temperature of 40.degree. C. owing to the setting of the valve 1. When the valve 1 is opened again after one minute, the overshoot, called after-boiling (owning to the residual heat of the heater), is caused as shown in FIG. 9, so that hot water of 50.degree. C. is temporarily disadvantageously outputted immediately after the valve 1 has been opened. The reasons are as follows.
When the valve 1 closes, the water flow within the tank 5 also stops. Then, although the pressure switch 2 cuts off the sheath heater 3, the water within the tank 5 continues to be heated by the residual heat of sheath heater 3. As there is no flow, the water is still and a water temperature distribution, as shown in the solid line of FIG. 10, with respect to the depth of the tank, results. Namely, the highest portion of the tank becomes about 50.degree. C. in temperature. A transition temperature gradient results between the upper portion and the lower portion of the tank so that the temperature decreases to about 25.degree. C., an inputwater temperature, near the input water opening 4a. As the hot water is outputted through the hot water output pipe 6 from the high-temperature water of the tank upper portion when the valve 1 is opened, the overheating of the output hot water temperature becomes large. It is natural that this tendency becomes greater as the water volume of the boiler becomes smaller.
In the abnormal condition (hereinafter referred to simply as "abnormal condition") where the sheath heater 3 remains on although valve 1 is enclosed, the water temperature within the tank 5 and the temperature of the sheath heater 3 rise. The thermostat 7 for preventing the excessive temperature rise operates to stop the energization of the sheath heater 3.
However, in such construction as described herein-above, it took a long time before the thermostat 7 for preventing the excessive rise of temperature responded to the abnormal condition. The resulting boiling water was jetted from the output hot water pipe 6, or, if boiling water was not discharged, the tank 5 was deformed by the resultant pressure buildup, thus resulting in a dangerous condition. The reasons are as follows.
Namely, the water temperature within the tank 5 near the temperature sensing portion 7a of the thermostat 7 during normal use is the highest of the water temperatures within the tank after the heating operation by the sheath heater 3. The temperature sensing portion 7a is normally kept highest in temperature by transfer heat from the U-shaped heater portion 3a. On the other hand, in the abnormal condition, the heat of the U-shaped heater portion 3a is robbed by the surrounding water, so that the temperature of sensing portion 7a is slow to rise in the abnormal condition. Also, the cut-off temperature of the temperature excessive-rise preventing operation is set with some tolerance (10.degree. C. or more), with respect to the highest temperature during normal use, for error prevention. The thermal transfer dispersion is caused because of the contact condition between the brazing or the like between the U-shaped heater portion 3a and the inner face of the tank 5, so that the tolerance is required. As shown in FIG. 11, the temperature of the heat sensing portion 7a of the thermostat 7 during normal use become higher as the output hot water volume becomes smaller, so that the operation-off temperature of the thermostat 7 has to be set at the high value. Thus, more time passes before the thermostat 7 takes the operationoff action in the abnormal condition, thus resulting in a dangerous condition such as boiling water within the tank 5, jetting from the output hot water pipe 6, or a deformed tank owing to pressure buildup.
Also, another embodiment of this type of conventional electric instantaneous boiler is shown in Japanese Patent Publication (Tokkosho) No. 59-53450, as in the construction of FIG. 8.
Namely, the temperature sensing portion 8a of a temperature detector 8, composed of a thermistor or the like for detecting the output hot water temperature, is provided in proximity to a mixing portion 10 for stirring the heated water of the upper portion of the tank 9 and the sheath heater 11. The water inputted into the lower portion of the tank 9 from the input water pipe 12 goes towards the upper portion of the tank 9 while being heated by the sheath heater 11, and is outputted from the output hot water pipe 13 after it has been stirred in the mixing portion 10. The temperature detector 8 detects the temperature of the water flowing to the mixing portion 10. The semiconductor power control apparatus 14 which inputted the detection signal compares the detected temperature value with the set temperature value to control in pulse the switching element 14a, such as a triac or the like, in accordance with the deviation valve so as to control the supply power to the sheath heater 11 so that the deviation value may be kept at zero. However, in such construction as described herein-above, the output hot water temperature becomes unstable with ripples being larger, as shown in B in FIG. 6, when the valve 15 is throttled to reduce the flow amount. The reasons are as follows.
Namely, when the flow amount is reduced, the flow speed near the temperature sensing portion 8a, which speed is high in the flow-passage area on the sectional face of the tank 9, becomes very slow. As the sheath heater 11 and the temperature sensing portion 8a are placed nearer each other for better thermal response resulting from a reduction of lag time, which is caused by the distance L of the temperature sensing portion 8a from the sheath heater 11, the temperature sensing portion 8a is influenced by the surface temperature of the sheath heater 11 to render the output hot water temperature stable.